Newfeature : Add selectable merhod

[wer-mathurin]

Add support for
https://api.flutter.dev/flutter/material/SelectableText-class.html

This is something very useful for flutter web.

[ReinBentdal]

Could you give an example on how you think this should work? I would gladly accept a pull request which implements this in a nice way!

[wer-mathurin]

@ReinBentdal I can imagine the same way as it work for the widget Text.
Currently I d not have time for PR. But, I usually do when I have time !
But I think commun use case is
const SelectableText(
'Hello! How are you?',
textAlign: TextAlign.center,
style: TextStyle(fontWeight: FontWeight.bold),
)

So if the widget is a SelectableText....using the usual styling method would be a great addition.
var selectable = SelectableText('SOME_TEXT).fontSize().textColor(...);

And nice to have:
var text = Text('SOME_TEXT').selectable(web: true, android: false, ios: false)